1. Field of the Invention
This invention relates to a method for preventing electrical charge exchange within an electrical system containing potential gradients (i.e. the voltage is not constant). Accordingly, leakage and noise currents are prevented from escaping or entering a conductor when the insulation thereof might be inadequate (e.g. particularly in wet environments or at high temperatures). A principal application of the present invention is to prevent leakage from or into a conductor that supplies current to or voltage from a remote sensor (e.g. a resistance temperature detector) where the calibration of the sensor depends upon current through or voltage across the sensor.
2. Background Art
Guarding is a well known technique for reducing the effects of electrical current leakage through electrical conductors as a consequence of imperfect insulation and shunt capacitance in low level and highly precise measurements and reducing common mode interference. These effects result from the finite impedance to ground and to nearby conductors that accumulate throughout a system.
Guarding has been traditionally applied to protect regions containing a single electrical potential. Driven guards typically take the form of an electrostatic enclosure and are often referred to as shields or guard shields. Any electrical circuit will necessarily develop voltage gradients when it is placed in operation. A non-self-generating voltage gradient example is a current carrying conductor that develops a potential gradient along its length. Some self-generating voltage gradient examples include parasitic potentials that develop within a guarded region via tribo and radiation-electric effects as well as the changing potential on mechanically variable cable capacitance holding an essentially constant charge. By way of further example, when using platinum resistance temperature detectors to obtain accurate measurements in high temperature environments, it was found that some temperatures were so high that available insulation compounds developed unacceptable levels of leakage. Such leakage adversely effected the accuracy of measurement and increased uncertainties relating thereto.
Accordingly, what is needed is a more effective and reliable guarding technique whereby almost any circuit or conductor can experience essentially no leakage in operation, even where poor insulation exists between the circuit and its environment and where parasitic electrical charges exist between a conductor and its environment. Reference may be made to the following U.S. patents which describe methods for guarding electrical regions:
______________________________________ 3,866,093 Kusters et al. 4,115,790 Tsunefuji 5,434,512 Schwindt et al. 5,457,398 Schwindt et al. ______________________________________